Cornerstone doesn't do svn merges. For me that's a deal killer (merging revisions from a a branch into the trunk? nope).
If all you need to do is commit and browse and checkout, Cornerstone is sexy and awesome, I like the UI better than Versions.
For daily usage -- branching, tagging, merging -- I like SmartSVN best and it's cross platform. UI isn't as sexy.
